Typical coin-counting machines discriminate coins by passing them by one or more sensors that read properties or characteristics of the coins, such as material or size characteristics. Generally, when a coin of a particular denomination is examined, the sensors return a reading for each coin characteristic of interest. A range of acceptable reading values (e.g., a “window”) can be defined for each coin characteristic of interest. For a particular coin to be accepted, each of the characteristic readings for that coin must fall within the defined window for that characteristic.
Determining the sizes of the windows often involves trade-offs between rejecting desirable coins that are on the margin and accepting undesirable (e.g., foreign or counterfeit) coins. As a result, the window sizes are often selected such that a portion of undesirable coins having characteristics close to the desirable coins will be accepted by the coin-counting machine. This raises the possibility of coin fraud by persons placing undesirable coins into the machine that have characteristics close to the characteristics of the desirable coins.
One method for preventing this type of coin fraud in coin-counting machines is to obtain a representative sample of the undesirable coin type that is being erroneously accepted, and adjusting the characteristics windows to exclude such coins. While this approach may be satisfactory for some coin types, it is often unsatisfactory for others because it can lead to an unacceptable rate of rejection of desirable coins. In addition, in some cases undesirable coins have characteristics that are so close to the desirable coins that it is difficult to exclude the undesirable coins by narrowing the windows of acceptability. As a result, a coin-counting machine may be able to reject a substantial portion of the undesirable coins, but enough of the undesirable coins are still accepted to encourage the defrauder to continue placing them in the coin-counting machine for credit. One method of addressing this problem has been to simply discontinue accepting the particular type of coin being defrauded. While this approach may be effective, it greatly reduces the benefits offered by coin-counting machines.